Monoclonal antibodies have been widely used as a scaffold for binding agents. The basic antibody structure will be explained here using as example an intact IgG1 immunoglobulin.
Two identical heavy (H) and two identical light (L) chains combine to form the Y-shaped antibody molecule. The heavy chains each have four domains. The amino terminal variable domains (VH) are at the tips of the Y. These are followed by three constant domains: CH1, CH2, and the carboxy-terminal CH3, at the base of the Y's stem. A short stretch, the switch, connects the heavy chain variable and constant regions. The hinge connects CH2 and CH3 (the Fc fragment) to the remainder of the antibody (the Fab fragments). One Fc and two identical Fab fragments can be produced by proteolytic cleavage of the hinge in an intact antibody molecule. The light chains are constructed of two domains, variable (VL) and constant (CL), separated by a switch.
Disulfide bonds in the hinge region connect the two heavy chains. The light chains are coupled to the heavy chains by additional disulfide bonds. Asn-linked carbohydrate moieties are attached at different positions in constant domains depending on the class of immunoglobulin. For IgG1 two disulfide bonds in the hinge region, between Cys235 and Cys238 pairs, unite the two heavy chains. The light chains are coupled to the heavy chains by two additional disulfide bonds, between Cys229s in the CH1 domains and Cys214s in the CL domains. Carbohydrate moieties are attached to Asn306 of each CH2, generating a pronounced bulge in the stem of the Y.
These features have profound functional consequences. The variable regions of both the heavy and light chains (VH) and (VL) lie at the “tips” of the Y, where they are positioned to react with antigen. This tip of the molecule is the side on which the N-terminus of the amino acid sequence is located. The stem of the Y projects in a way to efficiently mediate effector functions such as the activation of complement and interaction with Fc receptors, or ADCC and ADCP. Its CH2 and CH3 domains bulge to facilitate interaction with effector proteins. The C-terminus of the amino acid sequence is located on the opposite side of the tip, which can be termed “bottom” of the Y.
Two types of light chain, termed lambda (λ) and kappa (κ), are found in antibodies. A given immunoglobulin either has κ chains or λ chains, never one of each. No functional difference has been found between antibodies having λ or κ light chains.
Each domain in an antibody molecule has a similar structure of two beta sheets packed tightly against each other in a compressed antiparallel beta barrel. This conserved structure is termed the immunoglobulin fold. The immunoglobulin fold of constant domains contains a 3-stranded sheet packed against a 4-stranded sheet. The fold is stabilized by hydrogen bonding between the beta strands of each sheet, by hydrophobic bonding between residues of opposite sheets in the interior, and by a disulfide bond between the sheets. The 3-stranded sheet comprises strands C, F, and G, and the 4-stranded sheet has strands A, B, E, and D. The letters A through G denote the sequential positions of the beta strands along the amino acid sequence of the immunoglobulin fold.
The fold of variable domains has 9 beta strands arranged in two sheets of 4 and 5 strands. The 5-stranded sheet is structurally homologous to the 3-stranded sheet of constant domains, but contains the extra strands C′ and C″. The remainder of the strands (A, B, C, D, E, F, G) have the same topology and similar structure as their counterparts in constant domain immunoglobulin folds. A disulfide bond links strands B and F in opposite sheets, as in constant domains.
The variable domains of both light and heavy immunoglobulin chains contain three hypervariable loops, or complementarity-determining regions (CDRs). The three CDRs of a V domain (CDR1, CDR2, CDR3) cluster at one end of the beta barrel. The CDRs are loops that connect beta strands B-C, C′-C″, and F-G of the immunoglobulin fold. The residues in the CDRs vary from one immunoglobulin molecule to the next, imparting antigen specificity to each antibody.
The VL and VH domains at the tips of antibody molecules are closely packed such that the 6 CDRs (3 on each domain) cooperate in constructing a surface (or cavity) for antigen-specific binding. The natural antigen binding site of an antibody thus is composed of the loops which connect strands B-C, C′-C″, and F-G of the light chain variable domain and strands B-C, C′-C″, and F-G of the heavy chain variable domain.
The loops which are not CDR-loops in a native immunoglobulin, or not part of the antigen-binding pocket as determined by the CDR loops and optionally adjacent loops within the CDR loop region, do not have antigen binding or epitope binding specificity, but contribute to the correct folding of the entire immunoglobulin molecule and/or its effector or other functions and are therefore called structural loops for the purpose of this invention.
Prior art documents show that the immunoglobulin-like scaffold has been employed so far for the purpose of manipulating the existing antigen binding site, thereby introducing novel binding properties. In most cases the CDR regions have been engineered for antigen binding, in other words, in the case of the immunoglobulin fold, only the natural antigen binding site has been modified in order to change its binding affinity or specificity. A vast body of literature exists which describes different formats of such manipulated immunoglobulins, frequently expressed in the form of single-chain Fv fragments (scFv) or Fab fragments, either displayed on the surface of phage particles or solubly expressed in various prokaryotic or eukaryotic expression systems.
WO06072620A1 describes a method of engineering an immunoglobulin which comprises a modification in a structural loop region to obtain new antigen binding sites. This method is broadly applicable to immunoglobulins and may be used to produce a series of immunoglobulins targeting a variety of antigens. A CH3 library has been shown to be useful for selecting specific binders to an antigen.
Although multivalent display of proteins on genetic packages has been described (such as direct phage cloning and display, bacterial display, yeast display), prior art refers to monomeric monovalent display of binding domains, in general. WO9209690 describes phagemid particles displaying a single copy of a fusion protein on the surface of the particle. Thereby it was described to obtain high affinity binders from a library of phagemid particles, also called bacteriophages. Replicable expression vectors comprising genes encoding a binding polypeptide and a phage coat protein are provided so to form a gene fusion encoding a fusion protein, which is a chimeric protein of a phagemid particle, the phage coat protein and the binding polypeptide.
U.S. Pat. No. 5,223,409 generally describes the method of fusing a gene encoding a protein of interest to the N-terminal domain of the gene III coat protein of the filamentous phage M13. The gene fusion is mutated to form a library of structurally related fusion proteins that are expressed in low quantity on the surface of a phagemid particle. Biological selection and screening is employed to identify novel ligands useful as drug candidates.
However, there are some limitations in using such “fusion phage” or monovalent phage display and respective single fusion proteins. Many biologicals naturally occur in oligomeric form. For the purpose of the present invention oligomeric means dimeric, trimeric or even higher polymeric forms, up to 24 monomers.
The fusion phages according to the prior art are described to display monomeric fusion proteins, mainly because it was believed that binders of highest affinity could only be selected from a library if single fusion proteins are displayed by the phagemid particles. Native proteins are however often assembled as a dimer or even at a higher degree of oligomerization. To obtain dimeric display with a single fusion protein, some techniques have been developed that involve conditional stop codons located between the coat protein and the binding polypeptide (Dall'Acqua et al The Journal of Immunology, 2002, 169: 5171-5180). Thereby soluble monomers of the polypeptides in addition to those fused to the phage are expressed, thus enabling the formation of a dimer. However, such stop codons requires propagation in specific suppressor host cells that may translate a stop codon in an amino acid, to provide an appropriate amount of fusion proteins in addition to the soluble binding polypeptides. WO 03/029456 describes the use of multi-chain eukaryotic display vectors for the selection of immunoglobulin Fab fragments on the surface of yeast cells.
Prior art fusion proteins involve in some cases linker sequences to display larger binding polypeptides. Linker sequences of up to 24 amino acids are usually employed for standard purposes of displaying variable domains of an antibody. See for example, the display vector pCOMB3x (Hybrid. Hybridomics. 2003 April; 22(2):97-108. Development of functional human monoclonal single-chain variable fragment antibody against HIV-1 from human cervical B cells. Berry J D, Rutherford J, Silverman G J, Kaul R, Elia M, Gobuty S, Fuller R, Plummer F A, Barbas C F.)
It is an object of this invention to provide an effective method for the preparation of oligomers of modular antibody domains and to prepare such oligomers displayed on the surface of a replicable genetic package.